Gallium nitride (GaN) is a nitride semiconductor having wurzite structure, and has a direct transition type band gap of 3.4 eV corresponding to a blue wavelength range in visible light at room temperature, forms a homogeneous solid solution with InN and AlN to control a forbidden bandwidth, and exhibits characteristics of a direct transition type semiconductor in the entire composition range of a homogeneous solid solution. Thus, GaN is used the most widely as a material for a blue displaying and light emitting device.
Generally, a GaN single crystal is formed on a base substrate made of sapphire (Al2O3), silicon carbide (SiC) or silicon (Si) by metal organic chemical vapor deposition (MOCVD) or hydride vapor phase epitaxy (HVPE). However, the base substrate and a GaN film have different lattice constants and coefficients of thermal expansion, and thus a lattice mismatch makes it difficult to grow epitaxially the GaN film on the base substrate.
To overcome the problem, a technique was suggested to form a buffer layer having a similar lattice constant on a base substrate at a relatively low temperature and grow a GaN single crystalline layer on the buffer layer in order to relieve a lattice strain. However, this technique needs a high-cost base substrate and a growth apparatus for forming a buffer layer. And, this technique can grow epitaxially a GaN single crystalline layer, but exhibits a high dislocation density in the GaN single crystalline layer, and thus has limitations in application to a laser diode or an emitting diode.
According to modern technology, a GaN single crystalline layer can be formed on a sapphire base substrate relatively easily, but the GaN single crystalline layer should be split from the sapphire base substrate to obtain a free-standing GaN single crystalline substrate.
Splitting of a GaN single crystalline layer from a sapphire base substrate uses mechanical polishing or laser lift-off of the sapphire base substrate. The mechanical polishing polishes a sapphire base substrate to make the sapphire base substrate thinner, which results in destruction of an equilibrium reached immediately after growth of a GaN single crystalline layer, and consequently creation of cracks in the sapphire base substrate. The cracks propagate to the GaN single crystalline layer, which makes it difficult to obtain a high-quality large-area GaN single crystalline substrate. Meanwhile, the laser lift-off irradiates an ultraviolet laser on a sapphire base substrate to thermally decompose gallium nitride into gallium and nitrogen at an interface between the sapphire base substrate and a GaN single crystalline layer. However, the laser lift-off has disadvantages of long time and a low yield.
Alternatively, a silicon substrate may be used as a base substrate. This method can produce a large-area substrate at a low cost, and selectively etch and remove a silicon base substrate only. Thus, advantage is easier splitting of a substrate than use of a sapphire base substrate. However, it is still difficult to grow a GaN layer on a silicon base substrate, and the silicon base substrate may be etched during growth of the GaN layer. Further, although a GaN layer is grown on a silicon base substrate, bending and cracks may occur to the silicon base substrate due to differences in coefficient of thermal expansion and lattice constant between the silicon base substrate and the GaN layer.